Potentiometer



y 1951 J. G. BRADY E1" AL 2,559,394

POTENTIOMETER Filed May 15, 1950 1 IVENTOR JOSEPH G. BRADY CLYDE HJACKSON ATTORNEY Patented July 3, 1951 POTENTIOMETER Joseph G. Brady,Minneapolis, and Clyde H. J ackson, St. Paul, Minn., assignors toMinneapolis- Honeywell Regulator Company,

Minneapolis,

Minn., a corporation of Delaware Application May 15, 1950, Serial No.162,016

6 Claims. (Cl. 201-56) This invention relates to potentiometers andspecifically to non-inductive wire wound potentiometers. It is thereforean object of the invention to provide a new and improved type ofnoninductive wire wound potentiometer.

Another object of the invention is to provide a non-inductive wire woundpotentiometer in which the wiper arm may engage all the turns of theresistance winding portion.

Non-inductive potentiometers may be used to particular advantage inalternating voltage circuits in that there is no reactance across thepotentiometer. As a result, there would be no phase shift in the voltageacross the potentiometer which might be detrimental to the operation ofthe system. Further, such a non-inductive potentiometer could be used ina circuit wherein the alternating voltages vary over a range offrequencies without the eflective impedance of the potentiometerchanging. Because there would be no inductance in the potentiometer,there would be no reactance and the only impedance would be resistive,which does not change with change in frequency.

In one embodiment of the invention, the resistance unit for thepotentiometer comprises a single layer of wire of very low resistanceand an overlying layer of high resistance wire, the two layers beingwound so that their inductive effects are in opposition andsubstantially cancel each other out. The two windings are connectedtogether at one end and the opposite ends of the two windings serve asthe two terminals of the resistance unit. The relative resistance of thetwo windings is such that a very high percentage of the total voltagedrop is across the outer layer, and consequently, a wiper arm adjustableover the outer layer may select substantially any percentage of theterminal voltage.

The resistance unit for the potentiometer may also be made by firstconnecting one end of the conductor to one end of the resistance wireand then winding the conductor and resistance wire on the core togetherso that the turns on each side of the resistance wire are composed ofthe conductor and vice versa. Here again the turns of the conductor andresistance wire are parallel. In this case the resistance wire must beof larger diameter than the conductor in order that the portion of theturns of the resistance wire en- 'gaged by the wiper arm will be abovethe turns of the conductor so that the wiper arm will make contact onlywith the resistance wire.

Non-inductive potentiometers have been known in the art but prior to thepresent invention all such potentiometers used resistance wire for bothwindings on the core and as a result. the voltage obtainable on thewiper arm of the potentiometer was variable over only half the range ofthe voltage between the terminals of the potentiometer. By use of thepotentiometer herein disclosed, the voltage obtainable on the wiper armis capable of varying over substantially the entire range of the voltageimpressed across the potentiometer terminals. This results in a muchmore usable potentiometer and also requires only about half as muchvoltage on the terminals for the same voltage output as would berequired for potentiometers heretofore known.

For a better understanding of the invention reference is had to thefollowing detailed description taken in conjunction with theaccompanying drawing in which Figure 1 shows, partly in cross-section,one form of the complete resistance unit for the potentiometer;

Figure 2 shows the resistance unit of Figure 1 in the process of beingwound;

Figure 3 shows the resistance unit of Figure 1 farther along in theprocess of being wound than as shown in Figure 2;

Figure 4 shows a modification, partly in crosssection, of the resistanceunit for the potentiometer; and

Figure 5 shows the resistance unit of Figure 4 in the process of beingwound.

In the resistance unit of Figure 1, a core I0 of insulating materialshown to have a first layer of winding composed of a conductor I I withone end 12 of the conductor being connected, as by a soldered connection13 to a resistance wire I which is wound back over the conductorwinding. The end l5 of conductor H, which is opposite to soldered end l2and connection It of the resistance winding opposite the soldered end l3extend from the potentiometer and may be connected across a voltagesource to impress a voltage across the conductor and resistor windings.A wiper arm ll makes contact with the resistance winding and is movableacross the entire length of the resistor winding of the potentiometer.

For purposes of illustration, the size of the wire with relation to thediameter of the core has been greatly exaggerated, and the number ofturns has been greatly reduced. In actual practice, the ratio of coildiameter to wire diameter will be so large that the diflerence indiameters ofthe inner and outer layer will produce little difference ininductance of the two coils. However, if necessary, the inner layer maybe continued beyond the outer layer a short distance to bring itsinductance up to that of the outer layer.

This resistance unit of the potentiometer may be wound by fastening thefree end of the conductor to the core and then rotating the core, as isshown in Figure 2, and winding the conductor I l' upon thecore such thatthe adjacent turns of the conductor rest against each other. Theconductor must have an insulation, perhaps of varnish, so thatelectrical contact is not made between the adjacent turns but that anycurrent which passes through the conductor must of necessity flow alongthe conductor. The resistance wire must also be insulated in order thatthe current which flows through the resistance wire does not travel fromturn to turn but must rather flow along the resistance winding.

After the conductor is completely wound on the core it is connected tothe resistance wire H and the connection is then fastened to the core.The core is then rotated in the reverse direction and the resistancewire is wound on the core back over the conductor until the resistancewire is wound over the length of the core. The end of theresistance wiremay then be fastened to the core in any desirable manner in order thatthe wire may not be unwound from the core. When the potentiometer iscompletely wound, a surface of the resistance wire winding is thencleaned in order that the wiper arm may make contact with the bareresistance wire. Obviously, the resistance wire is not cleaned to suchan extent that the adjacent turns of the resistance wire may makeelectrical contact with the other but is only cleaned sufllciently topermit the wiper arm to make electrical contact with the turns.

In order to show how this type of winding elimnates inductance effect ofthe potentiometer let it be assumed that an alternating voltage isplaced across terminals l5 and I6 of the conductor and resistance wiresrespectively and then consider what happens on the half cycle when theconductor terminal I5 is positive with respect to resistance wireterminal l8. It can be seen by a simple application of the right handrule where the fingers point in the direction of current flow and thethumb points in the direction of the magnetic flux that to determine thedirection of the magnetic flux through the conductor the fingers willextend up behind the potentiometer, as it is shown in Figure 1 and thethumb will then be pointing toward the right. The current flow in theresistance wire on the other hand, will be flowing, during this halfcycle, such that the fingers of the right hand are extended downwardlybehind the potentiometer with the thumb then pointing toward the left.Thus the magnetic fluxes from the conductor and from the resistance wireare directly opposing each other and the magnetic fluxes are cancelledout. On the next hall. cycle the current flow through the conductor isinthe opposite direction so that the magnetic flux will be toward theleft. The current flow through the resistance wire is also in theopposite direction and the resulting magnetic flux will be toward theright and the magnetic fluxes will again cancel each other out. Thus, anon-inductive potentiometer has been produced.

Because half of the windings on the core are made from a conductorrather than all of the windings being composed of the resistance wire,virtually the entire voltage impressed across terminals l5 and It of theconductor and resistance wire is across the resistance wire which iscontacted by wiper arm I1. Thus, almost any portion of the voltageacross terminals l5 and it may be picked ofi by the wiper arm. If theentire windings were made from the resistance wire, the first layer ofwindings would be completely covered by the second layer and the wiperarms could not make contact to the first layer. Thus, the wiper arm llcould pick off only from half of the voltage across terminals 55 and I6to'all of the voltage across these two terminals, if the voltage pickedoff were between terminal i5 and wiper arm H, or. the voltage picked offby the wiper arm would be between zero voltage and a half of the voltageimpressed between terminals 15 and iii if the voltage picked off werethe voltage between terminal l6 and wiper arm l'l. If, then, it weredesired that the wiper arm be able to pick off voltages up to a certainmaximum voltage it would be necessary to impress twice that voltageacross the terminal, resulting in the use of higher voltages in thecircuit. Because of the possibility of electrical shocks, arcings, etc.,it is good practice to keep voltages in a circuit as low as possible inconformance with good performance by the circuit.

If desired, the resistance unit of the potentiometer may be wound asshown in Figures 4 and 5. In this case the core in has both theconductor and the resistance wire wound on the core at the same time. Inthe modification shown in Figure 4 the conductor Ila and resistance wirell are soldered together, as at 20, with these terminals then fastenedto the core. Figure 5, the core is then rotated and the conductor andresistance wire are fed onto the core such that the alternate turns arecomposed of conductor, resistance wire, conductor, resistance wire, etc.In this case, because the conductor and resistance wire both lie againstthe core and the turns are alternately resistance wire, conductor, etc.it is necessary that the conductor Ila. be of smaller diameter than theresistance wire ll in order that the wiper arm I! will make contact onlywith the resistance wire. Obviously, the conductor and resistance wireare again insulated so that there is no electrical contact between theadjacent turns. When the conductor and resistance wire have beencompletely wound on the core [0, they are fastened to the core by anysatisfactory means and the terminals |5a and I6 extend from theresistance unit. Again, one surface of the resistance wire is cleaned sothat the wiper arm I! may make contact with the turns of the resistancewire to pick off a voltage therefrom.

It can be seen that here again the windings of the conductor andresistance wire are opposite with respect to each other in that thewindings are parallel with respect to each other and the current flowthrough the turns is in the opposite direction. In this case theresistance wire must have a larger diameter than the conductor in orderthat the wiper arm will not make contact with the conductor but only tothe resistance wire. If the wiper arm were to make contact with theconductor it would, in time, scrape off the insulation on the conductorand thus make electrical contact with the conductor. The wiper arm is,of course, broad enough so as to make contact with two adjacent turns ofthe resistance wire simultaneously in order that the change in voltagepicked oil by the wiper arm changes smoothly with movement of the wiperarm along the resistance unit.

In order to show how the non-inductance is achieved in the potentiometerresistance unit as shown in Figure 4 let it be assumed that an As bettershown in.

alternatin voltage is impressed across terminals Iia and I6 of theconductor and resistance wire respectively and let the situation beconsidered on the half cycle that terminal I5a of the conductor ispositive with respect to terminal l6 of the resistance wire. On thishalf cycle by a simple use again of the right-hand rule the fingers ofthe hand will point down behind the potentiometer for the currentflowing through the conductor with the thumb pointing toward the left.The current flow through the resistance wire will be upwardly behind thepotentiometer and the thumb will point toward the right. Thus, themagnetic flux moving toward the left due to the current flowing throughthe conductor and the magnetic flux moving toward the right due to thecurrent flowing through the resistance Wire Will cancel each other outand a non-inductive potentiometer results. On the next half cycle, ofcourse, the magnetic fluxes due to the conductor and the resistance wireare in the opposite directions and will again cancel each other out.

In the potentiometers as shown in Figures 4 and 5 it is necessary torotate the core in only one direction in order to wind both theconductor and the resistance wires on the core. It is not possible withthis potentiometer resistance unit to place as many turns of theresistance wire on the core as is possible in the resistance unit asshown in Figure 1 and so a larger voltage drop is present across eachturn of the winding of the unit of Figure 4 than there is across eachturn of the unit shown in Figure 1, assuming in each case that the samemagnitude of voltages is impressed across the terminals and I5.

In the modification shown in Figure 4 it is also true, as was pointedout for the modification of Figure 1, that virtually the entire voltagedrop impressed across terminals I 5 and IE is across the resistance wireto which contact is made by the wiper arm 11.

While two desired forms of the invention have been shown and described,modifications within the spirit of the invention could be made by thoseskilled in the art and it is therefore to be understood that theinvention is to be defined and limited only to the extent of theappended claims.

We claim as our invention:

1. A potentiometer having an electrical conductor of comparatively lowresistance Wound in a first direction, a resistance wire ofcomparatively high resistance wound in the opposite direction over saidconductor such that the turns of the resistance wire are substantiallyparallel to the turns of the conductor, one end of the conductor beingconnected to one end of the wire, and a wiper arm making contact onlywith said resistance wire.

2. A potentiometer having a core, an electrical conductor ofcomparatively low resistance wound about said core in a first direction,a resistance wire of comparatively high resistance having one endconnected to one end of said conductor and wound about said core in theopposite direction such that the turns of the resistance wire aresubstantially parallel to the turns of the conductor, and a wiper armmaking contact only with said resistance wire.

3. A potentiometer having an electrical conductor of comparatively lowresistance wound in a first direction, a resistance wireof comparativelyhigh resistance having one end connected to one end of the conductor andwound in the opposite direction along said conductor such that the turnsof the resistance wire are substantially parallel to the turns of theconductor, and a wiper arm making contact only with said resistancewire.

4. A potentiometer having an electrical conductor of comparatively lowresistance wound in a. first direction about an insulated core, aresistance wire of comparatively highresistance having one end connectedto one end of the conductor and wound in the opposite direction aboutthe core, the number of turns of the resistance wire being the same asthe number of turns of the conductor, and a wiper arm making contactonly with said resistance wire.

5. A winding for a potentiometer having an electrical conductor ofcomparatively low resistance wound in a first direction; a resistancewire of comparatively high resistance having one end connected to oneend of said conductor and wound in the opposite direction along saidconductor such that the turns of the resistance wire are substantiallyparallel to the turns of the conductor, the periphery of the resistancewire being greater than the periphery of the conductor such that a wiperarm of the potentiometer can make contact only with the resistance wire.

6. A winding for a potentiometer having a core; an electrical conductorof comparatively low resistance wound about said core in a firstdirection; a resistance wire of comparatively high resistance having oneend connected to one end of said conductor and wound about said core inthe opposite direction along said conductor such that the turns of theresistance wire are substantially parallel to the turns of theconductor, the periphery of the resistance wire being greater than theperiphery of the conductor such that a wiper arm of the potentiometercan make contact only with the resistance wire.

JOSEPH G. BRADY. CLYDE H. JACKSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Germany Dec. 8, 193a of Commotion Patent No.2,5593% July 533 1951 JOSEPH G. BRADY ET AL.

It is hereby comifiexi that or gor appears in the printed specificationof the above numbored patent roo ulrlng correction as follows:

Column 2, line 39, for connection road and; line 40 for end readconnection;

and that the said Letters Patent should box-end as corrected above sothat the some may conform to the record of the case in the Patent Ofioo,

Signed and sealed this 9th day of Octoher A, D, 1951 THQMAS 1E. MURPHY,

Assistant 00mmosi0ner of Pafients.

